1. Field of the Invention
This invention relates to a process for electrostatic printing and more particularly, it relates to a process for electrostatic printing by using a master of high sensitivity, high durability, and panchromatic response.
2. Description of the Prior Art
Many printing methods are known. Among them, electrostatic printing methods belong to a special class. The principle of ordinary printing is based on selective application of ink onto the surface of a printing master due to uneven surface condition of the printing master or difference in solvent affinity, and transfer of the ink to a paper by a pressing action. On the other hand, in the electrostatic printing, the ink is replaced by a heat-fixable toner which is electrostatically adhered to a printing master, then transferred to and fixed on an image-receiving sheet, e.g., paper. While the ordinary printing has such as advantage that the ink is placed uniformly and firmly on the printing master to enable a large number of sheets of paper to be printed at high speed, it has such as a disadvantage that the ink is liable to adhere to portions of printing paper other than those to be printed. On the other hand, in the electrostatic printing methods, the toner can be adhered electrostatically so that firmness and uniformity of adhesion are heartily known dependent upon electrostatic "contrast" which is difficult to achieve, hence the method is not suitable for high speed printing, although staining of the printing paper as mentioned above is not so much problem as in ordinary printing. In view of the stated disadvantages, electrostatic printing has not been practically used as a clean printing method. In other words, electrostatic printing is poorer than conventional printing methods as to providing uniform and clear print in large number of sheets. For example, a representative electrostatic printing master which have been known is composed of a conductive support and an insulating image overlying the conductive support, or composed of an insulating support and a conductive image overlying on the insulating support. The image may be produced by applying an insulating or a conductive lacquer in the form of an image pattern onto the support, or by coating a photosensitive lacquer on a support, imagewise exposing and selectively removing the exposed or unexposed portions by etching. Such electrostatic printing masters have various drawbacks. For example, when it is used in the conventional electrostatic printing process sharpness of the printed image and durability of the master are usually poor. Such electrostatic printing process includes a charging step for forming an electrostatic image by selectively retaining electric charge at image portions, (the image portions are insulating), a developing step, in which a toner having a polarity opposite to that of the image portions is applied, and a transfer step, in which the toner image is transferred to a receiving sheet. For example, the known electrostatic printing master has images formed on its uneven surface, and such uneven surface is damaged by mechanical abrasion during the printing process to cause irregular charging, so that durability of the master is very low. Further more, it is very difficult to obtain a high resolving power with such uneven surface type master and also technically difficult to obtain a print having high resolution. Additionally, it is difficult to obtain images of half tone or gradation with such uneven surface type master.